Specific Aim #1: To conduct a multicenter Phase II, randomized, double-blind, placebo controlled trial to assess the impact of abatacept on the incidence of aGVHD and its biology. To make this assessment patients will be randomized either to standard aGVHD prophylaxis with a CNI, methotrexate and placebo or to investigational prophylaxis with a CNI, methotrexate and abatacept. Correlative immunology studies will be performed to elucidate abatacept's effects on the graft-versus-host response.